In prior designs for viscometers, a rotatably mounted container would hold the liquid whose viscosity is to be measured. A pendulum or a "bob" would be supported within the liquid on a wire, and the cylinder holding the liquid would be rotated. The twisting force imparted to the wire holding the bob would then be measured. This twisting force would be used as the basis for measurement of the viscosity of the fluid at a particular temperature. Bobs of the prior designs were generally supported at a single point. These prior designs would use thin wire, ranging from 0.002 to 0.010 inch in diameter, in combination with heavy bobs to keep the bobs centered in the sample when the rotating cup is moving. Other designs involved rods in lieu of thin wires, where the rods had a diameter greater than 0.125 inch. These designs lacked sensitivity because they required extreme forces to impart any twist to them. Yet, other designs, such as U.S. Pat. No. 4,299,118, employed rods in conjunction with a spring coupled to the drive through ball bearings. These types of connections had built-in inaccuracies because even with no sample in the rotating cup, a reading will be obtained when the drive is rotated, simply due to frictional forces between the bob and the rotating cup. The thin wire systems were limited to water and low-viscosity fluids. In the rod systems, inaccuracies occurred because the detection and force-measuring devices were not necessarily linear for the sample in the speed range of interest and because of bearing friction due to the mechanical method of construction.
Prior viscometers of the types discussed above are illustrated in U.S. Pat. Nos. 3,435,666; 1,281,042; 1,192,861; 1,236,706; 2,203,132; 2,303,162; 2,398,574; 2,957,339; and 4,242,086; European Patent Application Nos. 384,792; 007,427; 311,301; and 449,586; PCT Application Nos. WO92/10736; WO91/14168; and WO91/06364. Other relevant patents in the viscometer art include U.S. Pat. Nos. 4,299,119 and 3,435,665; and PCT Application No. WO92/06365.
While the prior designs described principally involved suspension of a bob in a cantilevered manner from a single or multiple suspension points, one prior design has incorporated suspension of the bob from a point removed from the fixation points for the torsion wire. In U.S. Pat. No. 2,796,758, the rotating cup 17 is specially constructed to have a U-shaped trough around its periphery. The torsion wire extends through the rotating cup and is fixed above and below the cup. The bob is, in turn, fixed to the torsion wire between the fixation points for the torsion wire. However, the readings of actual torsional displacement of the wire are taken adjacent the uppermost fixation point and not off the bob. A baseline system involving a target mounted to the bob for a neutral reading is used. Thereafter, a disc has to be physically rotated adjacent a fixation point until the indicator connected to the bob is again realigned with the target used to determine the initial position.
The apparatus of the present invention represents an improvement over the viscometer designs of the prior art, and, in particular, the Myers U.S. Pat. No. 2,796,758. The apparatus of the present invention involves a use of a standard rotating cup, rather than a specially designed cup, as used in Myers. The apparatus of the present invention provides multiple fixation points for the torsion wire, coupled with a suspension point for the bob assembly unit between the fixation points. In a preferred embodiment, only a single fixation point is used and tension is applied to the wire by virtue of concentric magnets which also centralize the bob. Both fixation points are aligned above the rotating cup so that standard cup geometries can be used. By fixing the bob as described, first-order inaccuracies due to deflection from a swinging bob are eliminated. Instead, the bob becomes self-centering through the use of the magnets, producing deflection inaccuracies of only the second order. The apparatus of the present invention is more sensitive because it can take the readings of deflection closer to the mid-point of the wire and further from either points of fixation. Using the magnets, the bob assembly can be suspended from different points on the wire without sacrificing accuracy. The apparatus of the present invention takes a direct readout of the rotation of the torsion wire and, through the use of electronic transducers to determine the amount of twist, eliminates another factor of human error in relying on visible deflections read off of an adjacent scale. The apparatus of the present invention involves simple and quick change-out of bobs with minimal effects on accuracy due to such change-outs.